This invention relates to a process for preparing halogenated organic compounds. More specifically, it relates to a process for preparing monohalogenated cyclobutarenes.
Monohalogenated cyclobutarenes are intermediates for the preparation of high performance monomeric and polymeric compositions for the electronics and aerospace industries. U.S. Pat. No. 4,540,763 discloses that monobrominated cyclobutarenes can be processed to prepare poly(cyclobutarene) polymeric compositions. These compositions possess thermal stability at high temperatures, as well as chemical resistance and low sensibility to water.
Processes for preparing monohalogenated cyclobutarenes are difficult because multiple halogenation reactions occur and the strained cyclobutane ring of the cyclobutarene is easily susceptible to ring-opening side reactions (see J. B. F. Lloyd et al., Tetrahedron, 20, pp. 2185-94 (1964)). U.S. Pat. No. 4,540,763 discloses a process for preparing monobrominated cyclobutarenes which involves diluting a cyolobutarene in acetic acid and then contacting the solution with pyridinium perbromide hydrobromide in the presence of a mercuric acetate catalyst. The reaction occurs over a four day period and uses approximately 300 percent excess brominating agent. J. B. F. Lloyd et al., Tetrahedron, 21, pp. 245-54, (1965), disclose a process for preparing monobrominated benzocyclobutene which involves diluting benzocyclobutene in a 95 percent aqueous solution of acetic acid and then contacting the solution with molecular bromine in the presence of an iodine catalyst. The yield of monobrominated benzocyclobutene is 78 percent after 48 hours. Unfortunately, both of these processes require large quantities of brominating agent to complete a very slow bromination reaction. Also, both processes require either a heavy metal catalyst or a halogen catalyst. The residual catalyst that inevitably finds its way into the final product is detrimental for electronics and aerospace industry applications. Furthermore, these catalysts create environmental problems related to their disposal.
Therefore, it would be desirable to have a process for preparing monohalogenated cyclobutarenes that does not require a halogen catalyst or a heavy metal catalyst. It would also be desirable to have a process providing a faster halogenation reaction highly selective to monohalogenated cyclobutarenes without requiring excessive quantities of halogenating agent.